The present invention relates generally to electrical switching devices. More particularly, the present invention relates to electrical switching devices that have a novel fastening arrangement for joining an electrical insulator, or a non-metallic tube, to a metal conductive flange within an electrical switching device using standard hardware.
A high voltage circuit breaker is a device used in the transmission and distribution of three phase electrical energy. When a sensor or protective relay detects a fault or other system disturbance on the protected circuit, the circuit breaker operates to physically separate current-carrying contacts in each of the three phases by opening the circuit to prevent the continued flow of current. In addition to its primary function of fault current interruption, a circuit breaker is capable of load current switching. A circuit switcher and load break switch are other types of switching device. As used herein, the expression xe2x80x9cswitching devicexe2x80x9d encompasses circuit breakers, circuit switches, dead tank breakers, live tank breakers, load break switches, reclosers, and any other type of electrical switch.
The major components of a circuit breaker or recloser include the interrupters, which function to open and close one or more sets of current carrying contacts housed therein; the operating mechanism, which provides the energy necessary to open or close the contacts; the arcing control mechanism and interrupting media, which interrupt current and create an open condition in the protected circuit; one or more tanks for housing the interrupters; and the bushings, which carry the high voltage electrical energy from the protected circuit into and out of the tank(s) (in a dead tank breaker). In addition, a mechanical linkage connects the interrupters and the operating mechanism.
Circuit breakers can differ in the overall configuration of these components. However, the operation of most circuit breakers is substantially the same. For example, a circuit breaker may include a single tank assembly which houses all of the interrupters. U.S. Pat. No. 4,442,329, Apr. 10, 1984, xe2x80x9cDead Tank Housing for High Voltage Circuit Breaker Employing Puffer Interrupters,xe2x80x9d discloses an example of the single tank configuration and is incorporated herein in its entirety by reference. Alternatively, a separate tank for each interrupter may be provided in a multiple tank configuration. An example of a prior art, multiple tank circuit breaker is depicted in FIGS. 1, 2, 3, and 4. Circuit breakers of this type can accommodate 72 kV, 145 kV, 242 kV, and 362 kV power sources.
The circuit breaker shown in FIG. 1 is commonly referred to as a xe2x80x9cdead tankxe2x80x9d because it is at ground potential. FIG. 1 provides a front view of a three phase or three-pole circuit breaker having three entrance bushing insulators, 10, 11, and 12, that correspond to each respective phase. The bushing insulators may be comprised of porcelain, composite, or a hardened synthetic rubber sufficient to withstand seismic stresses as well as stresses due to the opening and closing of the interrupter contacts within the device. In high voltage circuit breakers, the bushings for each phase are often mounted so that their ends have a greater spacing than their bases to avoid breakdown between the exposed conductive ends of the bushings.
The circuit breaker is comprised of three horizontal puffer interrupter assemblies enclosed in cylindrical tanks 15, 16, and 17. Current transformers assemblies 20 and 21 (referring to FIG. 3), which comprise one of more circuit transformer and their exterior housing, are located underneath the bushing insulators on the exterior of the breaker to facilitate their replacement in field. Current transformers 20 and 21 measure outgoing current.
FIG. 2 provides a side view of the three-pole circuit breaker of FIG. 1 that shows the corresponding exit bushing insulator, 13, of the interrupter assembly housed in tank 15. FIG. 2 illustrates how entrance bushing insulator 10 and exit bushing insulator 13 is associated with tank 15. The entrance and exit bushing insulators for the interrupters in tanks 16 and 17 (not shown in FIG. 2) are arranged in a similar fashion. The devices, illustrated in FIGS. 1 through 3, have 3 pairs of entrance and exit bushing insulators, or a total of 6 bushing insulators.
Referring to FIG. 1 and FIG. 2, the three interrupter tank assemblies are mounted on a common support frame 19. The operating mechanism that provides the necessary operating forces for opening and closing the interrupter contacts is contained within an operating mechanism housing or cabinet 18. The operating mechanism is typically mechanically coupled to each of the interrupter assemblies through a common linkage such as a drive cam. The operating mechanisms can be, but are not limited to, compressible springs, solenoids, hydraulic, or pneumatic-based mechanisms.
FIG. 3 is a partial, cross-sectional view of the interrupter assembly housed within cylindrical tank 15 and shown in FIG. 1 and FIG. 2. A typical circuit interrupter is comprised of stationary and movable contact assemblies 31 and 23, respectively. Entrance insulator bushing 10 houses a central conductor 22 which supports movable contact assembly 23 within conductive tank 24. Movable contact assembly 23 is affixed to an insulator tube 25 through which a linearly operating rod 26 extends. Rod 26 operates movable contact 27 between its open and closed position in a well-known fashion.
Exit insulator bushing 13 houses a central conductor 30 which is connected to the stationary contact assembly 31 and is also supported within conductive tank 24. An insulator tube 32 extends between the stationary contact assembly 31 and the movable contact assembly 23.
The interior volume of tank 24, as well as the entrance and exit insulating bushings 10 and 13, are preferably filled with an inert, electrically insulating gas such as SF6. The electrically insulating gas fulfills many purposes. The arcing contacts within both the stationary and movable contact assemblies are subject to arcing or corona discharge when they are opened or closed. Such arcing can cause the contacts to erode and disintegrate over time. Current interruption must occur at a zero current point of the current waveshape. This requires the interrupter medium to change from a good conducting medium to a good insulator or non-conducting medium to prevent current flow from continuing. Therefore, a known practice (used in a xe2x80x9cpufferxe2x80x9d interrupter) is to fill a cavity of the interrupter with an inert, electrically insulating gas that quenches the arc formed. During operation of the contacts in assemblies 23 and 31, a piston, which moves with the movable contact in assembly 23, compresses the gas and forces it between the separating contacts and toward the arc, thereby cooling and extinguishing it. The gas also acts as an insulator between conductive parts within housing 15 and the wall of tank 24.
Referring again to FIG. 3, the circuit interrupter assembly is comprised of a combination of insulating materials, such as insulator tube 25 and insulator tube 32, and conductive materials that are joined together. Because the insulating and conductive materials have varying strengths, it is difficult to secure these materials together without damaging the comparatively weaker insulator. The insulator tube within the electrical switching device is typically made of a weak, non-metallic material. This tube is then joined to a metal flange that is conductive and is relatively tough in comparison to the insulator tube. The joint formed between the weaker insulating tube and the rigid conductive flange experiences both compressive and tensile stresses due to inter alia, seismic events, high amperage, gas pressure within the circuit interrupter, shipping of the device prior to installation, thermal cycling, and the continuous operation of the device itself.
Prior methods for attaching the insulator tube to the conductive flange use chemical bonding or glue. Other joinder methods, such as welding or clamping, are ineffective due to the disparity of strength and material differences of the insulator tube and conductive flange. FIG. 4 provides a cross-sectional view of the chemical joinder of the insulating tube and conductive flange of an electrical switching device of the prior art. Referring to FIG. 4, insulator tube 33 is joined to a metal mounting piece or flange 34 (which is part of the stationary or movable contact assemblies) by glue joints 35. The glue joints are typically located at periodic intervals or annularly around the external surface of tube 33.
Chemical joinder methods present numerous manufacturing problems. These methods increase cycle time due to the additional steps needed to fully cure the glue or finish the joint. The time requiredxe2x80x94to preheat, clean, glue the insulating tube and conductive flange on one end, glue the insulating tube and conductive flange on the opposite end, and curexe2x80x94typically lasts one manufacturing workday and is a frequent source of production bottle-necks. Further, the method for gluing the assembly is prone to quality problems due to the difficulties in aligning the parts for the three different pole assemblies. The pole assemblies have to be correctly oriented for left, right, and center tank installations. Oftentimes, the operator errs in joining the insulator tube to the correct flange for the various installations. Thus, there is a fairly high rejection rate of assembly. Moreover, once the insulating tube and conductive, metallic flange are joined, the operator cannot disassemble and rework the joint or the assemblies contained therein without damaging the insulating tube. Lastly, there is also a safety and environmental concern because the workforce in the glue shop is exposed to potentially toxic chemicals as well as the high temperatures required for curing of the assembly.
The present invention provides electrical switching devices, more particularly an electrical switching device disposed between a pair of conductors, that have a novel fastening arrangement for directly and mechanically attaching an electrical insulating body, such as a non-metallic tube, to a conductive metal piece or flange within a conductive switching portion. The present invention uses standard hardware to provide a solid, reversible joint between a non-metallic, insulating body and a metal mating piece or flange that is subject to high voltage and high vibrational stress conditions. The fastening arrangement of the present invention reduces manufacturing cycle time, allows for manufacturing rework and reassembly, and minimizes the rejection rates associated with chemical bonding. Moreover, the joinder method of the present invention allows the insulating body and flange fit-up to have reasonable dimensional tolerances thereby reducing the cost and assembly difficulties of the final product due to the relative flexibility of the insulating tube material. The insulating body material deforms, or experiences localized crushing, at the areas where the fasteners are inserted. Because of this, a tight fit is achieved at the point of joinder.
According to the invention, the electrical switching device has a conductive switching portion, for carrying current between a pair of conductors, and a non-metallic, insulating body that comprises a hollow center in which a portion of the conductive switching portion contacts a portion of the insulating body. In preferred embodiments, a portion of the conductive switching assembly is inserted into the hollow center of the insulating body. The present invention may further comprise a conductive mounting piece, or flange, that is part of the switching device""s contact assemblies or a subset of the conductive switching assembly. In preferred embodiments, the insulating body is tubular shaped. The insulating body and conductive flange are held together at one or more ends of the insulating body by securing the insulating body to a conductive piece or flange via a plurality of fasteners. These fasteners collapse or deform a portion of the insulating body against the portion of the conductive switching portion that contacts the insulating body. The dimensional tolerances of the insulating body with respect to the conductive flange are chosen to pre-load the insulating body against the conductive flange.
The fasteners that engage the insulating body and conductive switching portion, such as a conductive mounting piece or flange, are within close proximity to one or more of the ends of the insulating body. Preferably, the distance between the fasteners and the end of the insulating body is no less than a multiplier of 2 times the diameter of one of the fasteners. In more preferred embodiments, this distance is a multiplier of 3 times the diameter of one of the fasteners.
In preferred embodiments, the fasteners are inserted into the exterior surface of the insulating body and protrude slightly. The fasteners are positioned equidistantly (or circumferentially if the insulating body is tubular) with respect to each other around the perimeter, or circumference, of the insulating body or tube to uniformly attach the insulating body to the conductive flange. Depending upon the desired strength of the joint, the distance between the fasteners can be, but is not limited to, between about 1-xc2xdxe2x80x3 to 2xe2x80x3, between about 2xe2x80x3 to about 3xe2x80x3, or between about 3xe2x80x3 to about 4xe2x80x3 for a high strength, medium strength, or low strength joint, respectively.
The number of fasteners is dependent upon the size of the insulating body or diameter of the insulating tube. For electrical switching devices of the present invention, the number of fasteners is preferably between 4 and 32, or more preferably between 8 and 16, to provide reliable performance and properly distribute the applied load.
The fasteners used to join the insulating body and conductive flange in the present invention can be standard hardware, such as, but not limited to, retention bolt and nut combinations, screws, screw and nut combinations, screw or bolt and washer combinations, or rivets. In preferred embodiments, the fasteners are flat-head, allen bolts, or conical shaped, flat-headed bolts, that are threaded to engage the insulating body. In other embodiments, the fasteners selected are bolts in combination with a special conical-shaped nut or washer with a threaded insert. The conical shape for the fasteners is preferred to radially compress the insulating tube at the cone shape of the bolt, to pre-load the insulating material, and to prevent material degradation at the interface of the fastener and insulating body when variable loads are applied to the insulating body.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. The accompanying drawings are included to provide a further understanding of the invention. In the drawings, like reference characters denote similar elements throughout several views. It is to be understood that various elements of the drawings are not intended to be drawn to scale.
A more complete understanding of the present invention, as well as further features and advantages of the invention such as its application to other electrical devices within a substation or system, will be apparent from the following Detailed Description and the accompanying drawings.